A recent increase in the awareness of energy saving demands highly efficient electric motors, and has led to many proposals for permanent-magnet-embedded electric motors achieving high efficiency by using rare earth magnets with high residual magnetic flux density and coercivity in a rotor. The permanent magnets are embedded in the rotor so as to be able to use not only magnet torque but also reluctance torque; therefore, the electric motor achieving high efficiency can be constructed. Because the reluctance torque is proportional to the difference between d-axis inductance and q-axis inductance, it is generally desired to have a structure that allows a q-axis magnetic flux to pass easily but does not allow a d-axis magnetic flux to pass easily in order to increase the reluctance torque. Here, the d axis is a radial axis passing through the center of the magnet and the q axis is an axis obtained by rotating the d axis by an electrical angle of 90°.
However, compared with a permanent-magnet-embedded electric motor not using the reluctance torque, the permanent-magnet-embedded electric motor with high usage of the reluctance torque is likely to have pulsation in a torque waveform and has an increased torque ripple. The torque ripple during operation of the permanent-magnet-embedded electric motor causes vibration and noise, and thus needs to be suppressed within a standard value.
Patent Literature 1 describes that the torque ripple is reduced by disposing a plurality of slits on the surface of a rotor in a permanent-magnet-embedded electric motor.